Design Method of Buck Switching Power Supply Based on Synchronous Rectification Technology
Typical Buck circuit
Buck circuit for synchronous rectification
Table of contents
Design Method of Buck Switching Power Supply Based on Synchronous Rectification Technology
3. Experimental test and simulation
Summary
As a basic switching power converter, the Buck converter is widely used in power conversion occasions. In order to solve the problem of large reverse current overshoot when the freewheeling diode is turned off when the Buck converter works in the continuous state of the inductor current, a synchronous rectification technology is adopted. The MOS tube is used to replace the freewheeling diode, and the control circuit outputs 180 ° complementary PWM waves to drive the switching MOS tube and the freewheeling MOS tube, which eliminates the reverse peak current at the end of the freewheeling, improves the efficiency of the Buck converter, and reduces The electrical stress of the small switch MOS tube reduces the electromagnetic interference of the whole device and improves the operation feasibility of the converter. The working principle of the Buck converter with synchronous rectification technology is analyzed, and its circuit simulation is carried out. The simulation results verify that the whole converter is feasible and practical.
Key words: Buck; freewheeling diode; peak current; synchronous rectification
0 Preface
Buck converter is a basic switching power converter, which has been widely used in small and medium power applications due to its simple structure, excellent performance, and small size. The freewheeling diode of the Buck converter performs freewheeling when the switch MOS tube is turned off, and the loss of the freewheeling diode affects the efficiency of the Buck converter to a certain extent. In the case of continuous inductor current, the turn-off current peak of the freewheeling diode increases the electrical stress of the filter and switch tube, thereby increasing the size and economic cost of the converter. Analyzing the working principle of the Buck converter and seeking a new freewheeling method play an important role in optimizing the structure of the Buck converter and reducing economic costs.
1 Principle Analysis
Buck converter is simple in structure, the main circuit is composed of switching MOS tube, freewheeling diode and LC low-pass filter, and its circuit structure principle is shown in Figure 1. The switch MOS tube is driven by PWM. When the switch MOS tube is turned on, the freewheeling diode D is cut off. When the switch MOS tube is turned off, the freewheeling diode is turned on.
- 1 Basic working principle
Assuming that all switching elements in the circuit are ideal components, the output filter capacitor C2 is large enough, and the inductance value of the filter inductor L is large enough, taking the continuous inductor current as an example for analysis, the voltage and current waveforms of the main components are shown in Figure 2.
During the 0-ton period, the switching MOS transistor is turned on, and the freewheeling diode is cut off. The current flowing through the switching MOS transistor is the inductor current. The current fluctuation of the filter inductor is mainly determined by the fluctuation of the switching MOS tube, and the main component of the loss of the converter is the switching loss of the switching MOS tube [5]. The voltage across the inductor is Vin-Vo, and the inductor current increases linearly.
During the ton-T period, the switch MOS tube is turned off, the freewheeling diode is turned on, and the current flowing through the freewheeling diode is the inductor current. The current fluctuation of the filter inductor is mainly determined by the fluctuation of the freewheeling diode, and the main component of the loss of the converter is the switching loss of the freewheeling diode [5]. The voltage across the inductor is reversed Vo, and the inductor current decreases linearly.
As can be seen above, the inductor current ripple is mainly caused by the fluctuations of the switching MOS tube and the freewheeling diode, and the main loss of the entire converter is the loss of the switching MOS tube and the freewheeling diode. Since the internal resistance of the switching MOS tube is generally small, which is much smaller than the loss of the freewheeling diode, it is necessary to discuss and improve the problems existing in the freewheeling process of the freewheeling diode.
1.2 Problems in the design process
The turn-off process of the freewheeling diode is analyzed, and there is a current reverse process in the turn-off process, and the voltage and current in the turn-off process are shown in Figure 3. In Figure 3, IF is the current flowing through the diode, UF is the voltage across the diode, and UR is the reverse voltage applied across the diode. It can be seen from Figure 3 that the current reverses first at time t1 during the turn-off process of the diode, and then the voltage reverses at time t3, and the current has a large reverse current at time t4, and resonates to zero at the last moment.
According to the dynamic characteristics of the freewheeling diode and the working principle of the Buck converter, there are the following problems:
1) From the dynamic process in Figure 3, it can be seen that the voltage and current are not zero at the same time during the diode turn-off process, plus the loss when the freewheeling diode is turned on, the entire diode has a large loss during the freewheeling process , affecting the Buck converter
s efficiency.
- When the inductor current is continuous, there is a large reverse current overshoot in the freewheeling diode, which is provided by the switching MOS tube, so the switching MOS tube needs to provide a large peak current instantaneously. While the peak current increases the electrical stress of the switch MOS tube, it is also easy to damage the switch MOS tube.
3) The peak current is large, the maintenance time is short, and the large didt will cause electromagnetic interference to the surrounding components, increase equipment noise, and reduce the reliability of the Buck converter.
Synchronous rectification technology In order to improve the efficiency of the Buck converter, reduce the electrical stress of the switch tube, reduce the volume of the filter, and optimize the performance of the Buck converter, it is necessary to optimize the freewheeling part. Since the loss of the switching MOS tube is small, the synchronous rectification technology is adopted, and another switching MOS tube is used to replace the freewheeling diode for freewheeling. The circuit principle is shown in Figure 4.
SG3525 is used as the control chip to output two complementary PWM waves with a phase difference of 180°. After passing through the bootstrap circuit with IR2110 as the core, it drives the switching MOS transistor Q1 and the freewheeling MOS transistor Q2 respectively. Two complementary PWM waves are shown in Figure 5.
In the 0-ton time period, the drive PWM wave controls the switch MOS tube to be turned on, and the freewheeling MOS tube is turned off. The current flowing through the switch MOS tube is the inductor current. In the ton-T period, the driving PWM wave controls the switch MOS tube to be cut off, the freewheeling MOS tube is turned on, and the current flowing through the freewheeling MOS tube is the inductor current. The use of MOS tubes instead of freewheeling diodes has the following advantages:
- The characteristics of the MOS tube are stable. Compared with the diode, the internal resistance is smaller, and the switching loss is smaller than that of the diode in the working process, which reduces the loss of the converter and is of great significance to the improvement of the efficiency of the Buck converter.
- The MOS tube does not need current during the shutdown process. When the inductor current is continuous, the MOS tube is used instead of the freewheeling diode to avoid a large reverse current overshoot at the end of the freewheeling, reduce the degree of sudden change in the current, and suppress electromagnetic interference. , to ensure the reliability of the equipment.
- Suppressing the peak current generation, reducing the electrical stress of the switching MOS tube, and reducing the ripple of the filter inductance, has a great effect on optimizing the size of the converter and the selection of devices, and reduces the cost of the converter.
2. Experimental test and simulation
When the Buck converter works in the continuous state of the inductor current, there is a large reverse current peak at the moment the freewheeling diode is turned off, resulting in a large peak of the MOS tube current at the turn-on moment. The buck circuit is tested with a rated value of 3 A/6 V, and the simulation results are shown in Figure 6. The peak current has a great influence on the circuit components.
In order to solve the problem of the reverse peak current generated during the turn-off process of the freewheeling diode, the synchronous rectification technology is used, and the circuit is tested. The waveform of the MOS tube is shown in Figure 7. Through the test, the peak current in the process of turning on the MOS tube disappears after adopting the synchronous rectification technology, and the dynamic performance of the switching device is good.
Before and after adopting the synchronous rectification technology, the ripple current parameters of the switching MOS tube and the freewheeling diode were compared in the circuit, and the half-load efficiency of the whole circuit was compared at the same time. The comparison results are shown in Table 1.
It can be seen from the results in Table 1 that after adopting the synchronous rectification technology, the freewheeling peak current value is reduced, the maximum value of the switching MOS tube current is reduced, and the overall efficiency of the Buck converter is greatly improved. After adopting the synchronous rectification technology, the performance of the Buck converter is greatly improved.
3 summary
In order to solve the problem of reverse peak current generated when the freewheeling diode is turned off when the Buck converter works in the continuous state of the inductor current, the synchronous rectification technology is adopted, and the MOS tube is used instead of the diode for freewheeling. The SG3525 chip is used as the control chip, and the switch MOS tube and the freewheeling MOS are driven through the bootstrap circuit with the IR2110 chip as the core. Through circuit principle analysis and circuit simulation, the results show that the Buck converter with synchronous rectification technology has the advantages of high conversion efficiency, small electromagnetic interference in the working process, and small electrical stress of switching MOS tubes, and has certain reliability and reliability in practical applications. Promotional.